The present invention relates to a method of determining a surface profile of an electrically conductive object. In the method a probe is used, which comprises a transmitter/receiver arrangement, wherein the transmitter is adapted to induce transient eddy currents in the object, and wherein the receiver is adapted to provide a signal indicative of the strength of a magnetic field or changes in the strength of a magnetic field. Transient eddy currents are generally induced when the electrically conductive object is subjected to a non-steady changing magnetic filed, such as generated by a coil that is energized by a current pulse. Transient eddy currents change with time after excitation, generally decaying to zero in the absence of further excitation, and so does the signal received in the receiver.
Transient eddy-current methods are useful inspection techniques, and examples of objects that can suitably be inspected with the method according to the present invention are metal plates or walls of container means, such as pipes, vessels or containers, having a radius of curvature that is larger than the thickness of the object. The electrically conducting material can be any electrically conducting material, for example carbon steel or stainless steel.
The term surface profile is used in the claims and in the description to refer to a representation of the shortest distance to a surface from known reference points, which are for example arranged along a line or in a plane substantially parallel to the surface to be inspected. A surface profile can reveal surface roughness, and anomalies such as a local decrease of the thickness of the object, which decrease is caused for example by corrosion.
A particularly important parameter which can be derived from a surface profile is the depth of an anomaly. When inspecting for example pipes, the depth of a corrosion spot is a critical parameter for the integrity assessment, since it is related to the remaining wall thickness.
Conventional methods for determining a surface profile are based on ultrasonic measurements at selected inspection points. Ultrasonic measurements cannot differentiate between an electrically conductive object to be investigated, and non-conductive materials covering the object, e.g. thermal insulation, paint, or corrosion products. It is therefore usually required to remove any such non-conductive materials, e.g. by grit blasting. For pipes and vessels operating at high pressures, this usually requires the equipment to be taken out of service, since safety considerations often forbid any in-service surface preparation. It may be particularly hazardous to remove corrosion products, since this may trigger a leakage. There is a need for novel non-invasive inspection methods, which would permit continued operation.
European patent specification No. 321 112 discloses a method of determining the thickness of a container wall means under insulation. In the known method, a probe is used comprising a transmitter coil for inducing eddy currents in the object, and a receiver system for providing a signal indicative of changes in the strength of a magnetic field. The known method comprises inducing transient eddy currents in the object; receiving a signal indicative of the eddy current, and comparing the decay of the received signal over a period of time with a reference decay indicative of a known wall thickness, whereby the thickness of the container means wall portion can be inferred.
When inspecting an object for corrosion, it is often not needed to actually determine the thickness of the object or of its wall. Rather, it will often be sufficient to inspect the surface that is accessible with a measuring instrument for irregularities and anomalies. This is particularly interesting for objects that have a substantially smooth surface profile at the time of installation, as is usually the case with pipelines or container walls. By measuring a surface profile later on, corrosion spots can be detected.
It would be advantageous to provide a new method for determining the surface profile of an electrically conductive object.
The invention relates to a method of determining a surface profile of an electrically conductive object, using a probe comprising a transmitter which is adapted to induce transient eddy currents in the object, and a receiver which is adapted to provide a signal indicative of the strength of a magnetic field or changes in the strength of a magnetic field, the transmitter and receiver forming a transmitter/receiver arrangement, according to the present invention comprises the steps of:
(a) selecting a calibration point on the surface, and selecting a number of calibration positions of the transmitter/receiver arrangement relative to the calibration point;
(b) determining a set of calibration values by determining, for each of the calibration positions, a characteristic value of the signal generated in the receiver in response to transient eddy currents induced in the object by the transmitter, wherein the characteristic value relates to the amplitude of the signal;
(c) determining a calibration function which relates the calibration values to the relative location of calibration position and calibration point;
(d) selecting a set of inspection points on the surface of the object, and selecting a set of inspection positions of the transmitter/receiver arrangement in correspondence to the set of inspection points;
(e) determining a set of inspection values by determining, for each of the inspection positions, a characteristic value of the signal generated in the receiver in response to transient eddy currents induced in the object by the transmitter; and
(f) determining the surface profile by interpreting the set of inspection values, wherein the calibration function is taken into account, and wherein the relative location of inspection points and corresponding inspection positions is derived.